It is well known in the field of injection molding that some means must be provided to inhibit the flow of molten material into the cavity of a mold so that the mold may be cooled and opened to remove the molded part. There are essentially two broad categories of inhibiting techniques known in the field of injection molds, namely: thermal gating, wherein the gate at the exit of the nozzle is rapidly cooled at the completion of the injection operation to form a solid or semi-solid plug of the material being injected at the gate; and valve gating, wherein a mechanical means is employed to inhibit the flow of material being injected into the mold cavity. Each category has its own advantages and disadvantages relative to the other and numerous systems for thermal gating and for valve gating are known.
Valve gating systems are generally of one of two types, namely inline and lateral systems, although a wide variety of systems of each type have been developed. In inline systems, a valve stem aligned with the gate is moved, parallel to the direction of movement of molten material (generally referred to as "melt") through the gate, between a position wherein the stem extends into the gate to block further flow through the gate and a position wherein the stem is retracted from the gate to permit flow therethrough into the mold cavity. In order to be aligned with the gate, the valve stem is located inside the injection nozzle and is at least partially within the flow path of the melt.
For these, and other reasons, inline valve gating suffers from a variety of problems. One common problem is wear of the valve stem due to contact with the nozzle and/or gate, which can lead to misalignment of the stem and thus leaking or failure of the valve. Another common problem is the conversion of the melt from the tubular flow entering the nozzle to an annular (or other non-continuous) flow which is caused by the valve stem or other related components being within the melt flow. Such a non-continuous flow can result in weld or knot lines in the molded product produced as the melt flow recombines within the gate or mold cavity and this can result in weakened or unacceptable molded products. Some examples of relatively recent inline systems which attempt to address these and/or other problems include U.S. Pat. Nos. 4,412,807 to York, 4,925,384 to Manner and 5,254,305 to Fernandez et al.
In lateral valve gating systems, a valve member is moved across the melt flow before or after the gate to block or allow flow through the gate. While lateral gating systems can avoid the misalignment, reliability and divided melt flow problems of inline valve gating systems, they suffer from their own problems and disadvantages. For example, a lateral valve gating system, which is located too far from the mold cavity, can result in unacceptable sprue vestiges being formed on the molded parts.
Another problem with gating systems in general is that the melt material in the melt channel of the mold cavity, adjacent the gate, experiences a different cooling regime than the remainder of the melt in the cavity. Specifically, as the nozzle is heated to maintain the melt in a molten condition, the melt material in the melt channel of the mold cavity adjacent the gate is cooled less efficiently than the rest of material in the mold cavity as some heat is transferred from the melt in the gate. Many melt materials degrade or otherwise develop undesired characteristics when exposed to a poor cooling regime. For example, in the specific case of PET, the material can exhibit crystalinity when exposed to a poor cooling regime and/or increased levels of acid aldehyde can be produced.
U.S. Pat. No. 4,108,956 to Lee shows, in the embodiments of FIGS. 1, 2 and 3, a lateral gating mechanism comprising a valve gate in the form of a pair of slidable plates with a bore there through. When the bore in these plates is aligned with the gate, melt material can flow into the mold cavity from the nozzle. When the injection operation is complete, the plates are moved laterally to move the bore out of alignment with the gate and further flow of melt into the cavity is prevented. The patent teaches that the advantage afforded by the plates is that they provide thermal insulation between the gate in the nozzle and the cooled mold to inhibit undesired thermal transmission therebetween, thus mitigating the poor cooling of the melt in the mold cavity adjacent the gate. Accordingly, the plates are of selected thickness and materials to provide the desired thermal insulating characteristics. In fact, the patent teaches that a pair of plates, each overlying the other, are employed with the plate adjacent the manifold plate being formed of a thermally insulating material and the plate adjacent the mold being formed of a heat conductive material.
However, as with many other lateral gating mechanisms, the device taught in Lee suffers from some associated disadvantages. In particular, some melt material is carried in the bore in the plates when they are moved to the position to inhibit melt flow and this material forms a cold plug therein. This results in two disadvantages, namely that some melt material is wasted on each closing operation and that some provision must be made to remove and discard the cold plugs from the bore before the plates are returned to the position to enable melt flow. Generation of waste is unacceptable in man) applications such as when molding PET preforms or in a clean room environment. The addition of the means to remove and discard the cold plugs is not acceptable in a multi-cavity mold. Therefore, the Lee patent does not represent a feasible approach.
U.S. Pat. Nos. 3,288,903 to Hendry, 3,599,290 to Garner, 3,632,729 to Biefeldt and 3,809,519 to Garner shown other examples of known lateral gating systems. These patents are mostly used to regulate the flow of melt from an injection machine. Therefore, the disadvantages of Lee '956, which are common for in-mold gating only, do not represent a major problem.
U.K. Patent 1,369,744, while not teaching a lateral gating system, does show in the embodiments shown in FIGS. 1 through 8 of the reference, a pair of valves which are located upstream from the gate and which comprise lateral shuttles. These shuttles are forced from the closed position (i.e.--where melt flow is inhibited) to the open position by pressure exerted by the melt, which counteracts an applied hydraulic pressure. The valves are used to switch materials to be injected, and are not used to control melt flow out of the gate and into the cavity. Further, as the gate is well downstream of the shuttles, a significant amount of melt is downstream of the shuttles and large, undesired, sprue vestiges can result on molded parts as this melt material is still in contact with the mold cavity. Also, these shuttles are actuated by a pressure differential between a hydraulic cylinder and the melt flows. This actuating mechanism cannot provide an accurate gating of multiple nozzles and would not be suitable for location within an injection mold due to the volume occupied by the mechanism and as hydraulic leaks, which are inevitable with prolonged use, are not tolerable in a mold.
Another problem of all conventional gating systems is that the gate is of a constant size. Presently, in all of the gating systems of which the present inventor is aware, the cross-sectional area of the gate is constant and thus limits the rate of melt flow into a mold cavity. In some circumstances, it is desired to inject different materials and/or different amounts of those materials into the mold cavity, such as in coinjection situations. In such cases, the mold designer must select a gate size, which is a compromise between the optimal size for each material and/or amount of material.
In other circumstances, it is desired to inject a material or materials at different rates. For example, a cantilevered mold core such as that used in molding blow molding preforms from PET can be shifted laterally in a mold cavity by the melt material which enters the cavity at the beginning of the injection operation. In conventional systems, the cavity is filled at a substantially constant rate, as the cross-sectional area of the gate and the feed pressure of the melt from the injection machine are substantially constant. Accordingly, melt material enters the cavity at a substantially constant pressure and velocity. If the mold gate size could be altered, melt could initially enter the cavity at a reduced pressure and/or velocity until some melt surrounds a portion of the mold core and then the gate can be resized to allow the remaining melt to enter at a higher pressure and/or velocity.
Also, with the prior art gating systems when a mold cavity is changed to mold a different part, it can be required to change the gates of the mold to larger or smaller gates to accommodate the new melt flow requirements. If the size of the mold gate in the mold could be altered, the time required to change the cavities could be reduced.
It is desired to have an apparatus and method of lateral gating for use in injection molding operations which provides the advantages of lateral gating without at least some of the drawbacks normally associated therewith. Further, it is desired to have an apparatus and method of lateral gating which permits the cross-sectional area of the gate to be altered. This alteration of the cross-sectional area of the gate allows melt to be injected under different conditions, such as different flow rates and/or pressures.